Sliding vane pumps are well known. They typically find favour in a wide variety of applications, such as ambient carbonation systems for producing beverages, commercial espresso coffee making equipment and cooling equipment in which a coolant is circulated. These diverse applications have in common the need for a positive displacement pump having a relatively high working pressure and flow rate, together with a long, maintenance-free operational life. These needs are met by the sliding vane pump.
Although the characteristics and performance of sliding vane pumps are adequate in many respects, there remains scope for improvement. For example, the housings of many sliding vane pumps are formed of brass alloys containing small amounts of lead. However, the use of lead in the pump housing is generally undesirable and may render a pump unsuitable for use in potable water applications, including the ambient carbonation systems and espresso coffee making equipment mentioned hereinabove. It is known to avoid this problem by forming the pump housing from stainless steel, but this comes at significantly increased cost. Plastics materials have also been proposed, but the associated moulding processes and strength issues introduce further complexity.
Another performance issue relates to the operating of sliding vane pumps in the so-called bypass mode. Sliding vane pumps are usually provided with a bypass valve which allows the pumped fluid to be transferred from an outlet to an inlet of the pump when the pressure at the outlet exceeds a predetermined level. In many applications, such as the ambient carbonation systems and espresso coffee making equipment mentioned hereinabove, the pump operates in the bypass mode for prolonged periods of time. Such operation can cause a build up of heat in the pumped fluid which, in turn, causes thermal expansion of pump components together with increased wear and premature failure of the pump. The inventors have identified a particular failure mode whereby thermal expansion of the rotor causes the vanes to become jammed in their slots.